UNIX System V
|Company / developer||AT&T Corporation|
|Source model||Closed source|
|Unixes by Bell|
UNIX System V, commonly abbreviated SysV (and usually pronounced—though rarely written—as "System Five"), is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph (AT&T) and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4. System V Release 4, or SVR4, was commercially the most successful version, being the result of an effort, marketed as Unix System Unification, which solicited the collaboration of the major Unix vendors. It was the source of several common commercial Unix features.
While AT&T sold their own hardware that ran System V (see AT&T Computer Systems), most customers ran a version from a reseller, based on AT&T's reference implementation. A standards document called the System V Interface Definition outlined the default features and behavior of implementations. The most widely used versions of System V today are IBM's AIX, based on System V Release 3, and Sun's Solaris and Hewlett-Packard's HP-UX, both based on System V Release 4.
In the 1980s and early-1990s, System V was considered one of the two major "flavors" of UNIX, the other being Berkeley Unix (BSD). During the period of the Unix wars System V was known for being the primary choice of manufacturers of large multiuser systems, in opposition to BSD's dominance of desktop workstations. However, with standardization efforts such as POSIX and the commercial success of Linux, this generalization is not as accurate as it once was.
System V, known inside Bell Labs as Unix 5.0, succeeded AT&T's previous commercial Unix called System III in January, 1983. There was never an external release of Unix 4.0, which would have been System IV. This first release of System V (called System V.0, System V Release 1, or SVR1) was developed by AT&T's UNIX Support Group (USG) and based on the Bell Labs internal USG UNIX 5.0. System V also included features such as the vi editor and curses from release 4.1 of the Berkeley Software Distribution of UNIX developed at the University of California, Berkeley (UCB); it also improved performance by adding buffer and inode caches. System V ran on the DEC VAX and PDP-11 machines. It also added support for inter-process communication using messages, semaphores, and shared memory, developed earlier for the Bell-internal CB UNIX.
System V Release 2 was released in April, 1984. It added shell functions and the SVID. New kernel features included record and file locking, demand paging, and copy on write. The concept of the "porting base" was formalized, and the DEC VAX 11/780 was named for this Release. The "porting base" is the so-called original version of a Release, from which all porting efforts for other machines emanate. Maurice J. Bach's The Design of the UNIX Operating System is the definitive description of the System V Release 2 kernel.
System V Release 3 was released in 1986. It included STREAMS, the Remote File System (RFS), the File System Switch (FSS) virtual file system mechanism, a restricted form of shared libraries, and the Transport Layer Interface (TLI) network API. The final version was Release 3.2 in 1988, which added binary compatibility to Xenix on Intel platforms; SCO OpenServer was based upon 3.2, as was Interactive Systems Corporation (ISC) 386/ix. Among the more obscure (re)distributions of SVR3.2 for the 386 were Esix 3.2 by Everex and "System V, Release 3.2" sold by Intel themselves; these two shipped "plain vanilla" AT&T's codebase.
User interface improvements included the "layers" windowing system for the DMD 5620 graphics terminal, and the SVR3.2 curses libraries that offered eight or more color pairs and other at this time important features (forms, panels, menus, etc.). The AT&T 3B2 became the official "porting base". IBM's AIX operating system is an SVR3 derivative.
System V Release 4.0 was announced on October 18, 1988 and was incorporated into a variety of commercial Unix products from early 1989 onwards. A joint project of Unix System Laboratories and Sun Microsystems, it combined technology from Release 3 as well as 4.3BSD, Xenix, and SunOS. New features included:
- From BSD: TCP/IP support, sockets, UFS, support for multiple groups, C shell;
- From SunOS: the virtual file system interface (replacing the File System Switch in System V release 3), NFS, new virtual memory system including support for memory mapped files, an improved shared library system based on the SunOS 4.x model, the OpenWindows GUI environment, External Data Representation (XDR) and ONC RPC;
- From Xenix: x86 device drivers, binary compatibility with Xenix (in the x86 version of System V);
- Korn shell;
- ANSI X3J11 C compatibility;
- Multi-National Language Support (MNLS);
- Better internationalization support;
- An application binary interface (ABI);
- Support for standards such as POSIX and X/Open.
The primary platforms for SVR4 were Intel x86 and SPARC; the SPARC version, called Solaris 2 (or, internally, SunOS 5.x), was developed by Sun. The relationship between Sun and AT&T was terminated after the release of SVR4, meaning that later versions of Solaris did not inherit features of later SVR4.x releases. Sun would in 2005 release most of the source code for Solaris 10 (SunOS 5.10) as the open source OpenSolaris project, creating the only open-source (heavily modified) System V implementation available. The "Principal Publisher" for SVR4 on the Intel platform was ISC.
Many versions of SVR4 appeared, because of hardware vendors (HP, SGI, NEC, Sony, OKI, Fujitsu) adapting it to their platform, and because porting houses (SCO, Microport, ESIX, UHC) sold enhanced and supported x86 versions. SVR4 was even ported to the Amiga as Amiga Unix and to the Atari TT030 as Atari System V. Other popular SysV derivatives include Dell SVR4 and Bull SVR4.
A consortium of Intel based resellers including Unisys, ICL, NCR Corporation, and Olivetti developed SVR4.0MP with multiprocessing capability (allowing system calls to be processed from any processor, but interrupt servicing only from a "master" processor). Release 4.1 ES (Enhanced Security) added security features required for Orange Book B2 compliance and Access Control Lists and support for dynamic loading of kernel modules.
SVR4.2 / UnixWare
In 1992, AT&T USL engaged in a joint venture with Novell, called Univel. That year saw the release System V.4.2 as Univel UnixWare, featuring the VERITAS File System. Other vendors included UHC and Consensys. Release 4.2MP, completed late 1993, added support for multiprocessing and It was released as UnixWare 2 in 1995.
The Santa Cruz Operation (SCO), owners of Xenix, eventually acquired the UnixWare trademark and the distribution rights to the System V Release 4.2 codebase from Novell, while other vendors (Sun, IBM, HP) continued to use and extend System V Release 4. Novell transferred ownership of the Unix trademark to The Open Group. Any operating system that meets the Single Unix Specification (SUS), effectively a successor to the System V Interface Definition, may be granted Unix rights. The SUS is met by Apple's Mac OS X, a BSD derivative, as well as several other operating systems not derived from either BSD or System V.
System V Release 5 was developed in 1997 by the Santa Cruz Operation (SCO) as a merger of SCO OpenServer (an SVR3-derivative) and UnixWare, with a focus on large-scale servers.:23,32 It was released as SCO UnixWare 7. SCO's successor, The SCO Group, also based SCO OpenServer 6 on SVR5, but the codebase is not used by any other major developer or reseller.
Decline on x86 platforms
In the 1980s and 1990s, a variety of SVR4 versions of Unix were available commercially for the x86 PC platform. However, the market for commercial Unix on PCs went into slow decline after Linux and BSD became widely available. In late 1994, Eric S. Raymond discontinued his "PC-clone UNIX Software Buyer's Guide" on USENET, stating, "The reason I am dropping this is that I run Linux now, and I no longer find the SVr4 market interesting or significant." In 1998, a confidential memo at Microsoft stated, "Linux is on track to eventually own the x86 UNIX market," and further predicted, "I believe that Linux – moreso than NT – will be the biggest threat to SCO in the near future."
Proprietary server platforms
The principal System V derivatives that remain in commercial use are AIX, HP-UX, and Solaris.
OpenSolaris and Illumos
System V compatibility
The System V interprocess communication mechanisms are even available in Unix-like operating systems not derived from System V; in particular, in Linux (a reimplementation of Unix) as well as the BSD derivative FreeBSD. POSIX 2008 specifies a replacement for these interfaces.
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